Blackbody theory
Place two objects in a void of empty space, one object hot the other cold. The two objects will exchange energy and eventually come to the same temperature. It is the mechanism of this exchange that is the basis of blackbody theory.
Under this theory the temperature of the hot object is a measure of the kinetic energy of the random vibrational motion of its’ atoms. As the atoms vibrate they drag their electrons with them. The moving electrons cause a disturbance in the electromagnetic field, draining energy from them. The emitted energy from the disturbed field is called blackbody radiation and is described by “Planks law”. Because the electrons are bound to matter, not all vibrational frequencies are generated in equal amounts, so only a certain quantity of energy at each wavelength is radiated. This quantity is precisely determined by the temperature, and is shown in the blackbody curve. This is why a blackbody can be used as a precision radiation source for instrument calibration.
Figure 1. Blackbody curve (from Wikipedia). As the object’s temperature increases the total amount of radiated power increases, and the energy distribution shifts toward shorter wavelengths. The power (photons/sec) at each wavelength is precisely calculable from Plank’s law. For this reason a blackbody makes an excellent standard source for light, and the amount of light is indicated by temperature (color temperature).